Determination Of Crop Evapotranspiration And Crop Water Demand Calculation Method

The study of evapotranspiration is not only the important component of water balance, but also the premise of making irrigation plan and evaluting climate resource and waler supplydt is important for agricultural production. Precisely measuring soil evaporation is important for improving water use efficiency.In this study,the large-scale weighing lysimeter and microlysimter are employed to determine the diurnal changes of evapotranspiration and evaporation in growing stages of winter wheat and summer maize.The ratios of soil evaporation to evapotranspiration in each growing stages are analyzed.The effects of soil moisture and leaf area index on the ratios of soil evaporation to evapotranspiration,and the changes of soil evaporation after irrigation are also analyzed.The daily changes of evapotranspiration and evaporation are measured.The law of ratios of transpiration to evaporation and the effect of meteorological factors on daily evapotranspiration are analyzed.The evaporation are measured using rnicrolysimters of two different materials and different specifications.The differences and reasons are arialyzed.The crop water requirement of winter wheat and summer maize are computed by single and dual crop coefficient approaches and compared with measured by Iysimeter.The differences and reasons are analyzed.The peak consumption of evapotranspiration of winter wheat is after turing green,particularly heading to milk stage.The main consumption of soil evaporation is before living through the winter.Evapotrarispiration peak of summer maize is after jointing.Soil evaporations are mainly affected by surface soil moisture.Evapotranspiration change from the development process.Leaf area index and surface soil moisture are two basic factors of controlling evaporation.The ratios of evaporation to evapotranspiration decrease with the increase of leaf area index .After full groudcovered, the ratios of evaporation to evapotranspiration increase with the increase of surface soil moisture.The ratios of evaporation to evapotranspiration are large in the intial stages.As the canopy development,the ratios of evaporation to evaotranspiration slowly become small.Solar radiation is main factor to affect daily evapotranspiration.It is a simple,effective and dependant method for using microlysimeter to measure evaporation.But different materials and specifications affect the measurement of evaporation.Cappirig the microlysimter bottom also affect evaporation,but the row position is no effect to evaporation.Crop water requirements computed by dual crop coefficient approach are better than by single crop coefficient approach under partly groud cover condition,but the differences of crop water requirements estimated from two approaches are very small under full ground cover condition.In the whole,the two approaches have good relations.Single crop coefficient approach is simpler than dual crop coefficient approach.It is suggested to using single crop coefficient approach for calculating crop water requirements of growth stages.But dual crop coefficient approach is recommended for irrigation prediction and policy decision.